January 10, 2006
10:30 AM (EST)

News Release Number: STScI-2006-04

Monster Black Holes Grow After Galactic Mergers

The full news release story:

An analysis of the Hubble Space Telescope's deepest view of the
universe offers compelling evidence that monster black holes in
the centers of galaxies were not born big but grew over time
through repeated galactic mergers.

"By studying distant galaxies in the Hubble Ultra Deep Field (HUDF),
we have the first statistical evidence that supermassive black-hole
growth is linked to the process of galaxy assembly," said astronomer
Rogier Windhorst, of Arizona State University in Tempe, Ariz., and
a member of the two teams that conducted the analysis. "Black holes
grow by drawing in stars, gas, and dust. These morsels come more
plentifully within their reach when galaxies merge."

The two teams will present their results in a press conference on
Jan. 10 at the 207th meeting of the American Astronomical Society
in Washington, D.C.

The HUDF studies also confirm the predictions of recent computer
simulations by Lars Hernquist, Philip Hopkins, Tiziana di Matteo,
and Volker Springel of the Harvard Smithsonian Center for
Astrophysics in Cambridge, Mass., that newly merging galaxies are
enshrouded in so much dust that astronomers cannot see the
black-hole feeding frenzy. The computer simulations, as supported
by Hubble, suggest that it takes hundreds of millions to a
billion years before enough dust clears so that astronomers can
see the black holes feasting on stars and gas from the merger. The
telltale sign that black holes are dining is seeing light from
galaxies that varies with time.

The two HUDF teams believe they are seeing two distinct phases in
galaxy evolution: the first phase - the tadpole stage -
representing the early-merging systems where central black holes are
still enshrouded in dust, and the much later "variable-object phase,"
in which the merged system has cleared out enough gas for the inner
accretion disk around the black hole to become visible.

"The fact that these phases were almost entirely separate was a
surprise, because it is commonly believed that galaxy mergers and
central black-hole activity are closely related. Our nearby universe
has mature galaxies, but in order to understand how they formed and
evolved, we must study them over time," Windhorst explained. "The
HUDF provides an actual look back in time to see snapshots of early
galaxies so that we can study them when they were young."

A link between the growth of galaxies through mergers and the feeding
of the central black holes has long been suspected. The evidence,
however, has been inconclusive for many years. "The HUDF has provided
very high-quality information. It is the first data we could use to
test this theory, since it allowed us to study about 5,000 distant
galaxies over a period of four months," said Seth Cohen of Arizona
State University and leader of one of the teams.

The HUDF observations have now shed light on how the growth of monster
black holes kept pace with that of galaxies. A team of astronomers,
led by Amber Straughn of Arizona State University, searched the HUDF
for "tadpole galaxies," so-called because they have bright knots and
tails caused by mergers. These features are produced when the galaxies
lose their gravitational grip on their stars, spewing some of those
stars into space. The team found about 165 tadpole galaxies,
representing about 6 percent of the 2,700 galaxies in the tadpole
galaxy study.

"To our surprise, however, these tadpole objects did not show any
fluctuation in brightness," Straughn said. "The flickering light 
when it is present  comes from the material swirling around an
accretion disk surrounding a black hole. The material is heated and
begins to glow. As it spirals down toward the black hole, it can
rapidly change in brightness. This study of tadpole galaxies suggests
that black holes in newly merging galaxies are enshrouded in dust,
and therefore, we cannot see them accreting material."

Cohen's team studied the brightness of about 4,600 HUDF objects over
several weeks to many months. The Hubble team found that about 45
(non-tadpole) objects, representing 1 percent of the faint galaxies
in the study, fluctuated significantly in brightness over time. This
result indicates that the galaxies probably contain supermassive
black holes that are feeding on stars or gas.

"A black hole's typical mealtime lasts at least a few dozen million
years," Windhorst said. "This is equivalent to black holes spending
no more than 15 minutes per day eating all their food - a veritable
fast food diet."

The HUDF analysis also reinforces previous Hubble telescope studies of
monster black holes in the centers of nearby, massive galaxies. Those
studies showed a close relationship between the mass of a galaxy's
"central bulge" of stars and that of the central black hole. Galaxies
today have central black holes with masses ranging from a few million
to a few billion solar masses.